1. Field of the Invention
The invention relates to a method and composition for effecting the cleaning of an isolated section of a hydrocarbon transmittion conduit, such as a pipeline, to remove paraffin deposits therefrom by chemically effecting the dissolution of the paraffin deposits.
2. History of the Prior Art
As used herein, "hydrocarbon transmission conduits" refers to any cylindrical means for transmitting hydrocarbons above the top surface of a subterranean well, such as means extending from the Christmas tree to and through the sales line, pipelines, conduits transmitting hydrocarbons to and within refineries, and the like. The accumulation of paraffin deposits in hydrocarbon transmission conduits, particularly subsea pipelines, is a problem that has been plaguing the oil-producing industry. By reference to "paraffin", we mean a wax-like organic precipitate from hydrocarbons of the alkane group. These paraffinic waxes are either normal, branched, or cyclic alkanes and are described by the following formula: C.sub.n H.sub.(2n+2), where n is a positive integer. These paraffins are generally inert to acids, bases, and oxidizing agents. Typically, when a crude oil containing paraffins flows through pipelines at comparatively reduced temperatures, such as would be encountered in a subsea pipeline, the paraffins will start precipitating out of the crude in the form of tiny crystals. As these paraffin crystals fall out of solution, they will adhere to the walls of the conduit, and the fluid passing through is exposed to an ever increasing number of nucleation sites. Contemporaneously, the rate of paraffin precipitation generally accelerates because of the abundance of these nucleation sites. Eventually, the effective flow area of the conduit is significantly reduced and, hence, cleaning of the pipeline to eliminate the paraffin deposits is desired.
Prior art methods of removing paraffin deposits from pipelines have been expensive and time-consuming. One method has employed a mechanical action by using "pigs" which effect a wiping action with the bore of the pipeline. Such pigs have been pumped through the partially clogged section of the pipe to mechanically remove the paraffins from the walls and then force the paraffins out to an accesible portion of the pipeline where the paraffins can be removed from the pipeline and properly disposed. However, this method is somewhat limited and is particularly effective only for the removal of a relatively small thickness of deposits of paraffins. As the paraffin deposits increase in thickness, the pressure required to force the pigs through the pipeline goes up substantially, sometimes approaching the pressure limits of the pipeline, and certainly the operator cannot afford to bulge the pipeline and thus destroy the external and/or internal coating of the pipeline.
A second prior art approach to removing deposited paraffins from the bore of pipelines and similar conduits used to transmit hydrocarbons is to employ a heated solvent capable of dissolving the paraffin deposit and to flow the heated solvent over the paraffin. This method is characterized by extremely high costs, both for the solvent solution and for the energy costs involved in maintaining the solvent at a sufficiently high temperature to effect the efficient dissolving of the paraffin deposit.
In U.S. Pat. No. 4,178,993 to RICHARDSON, et al, there is disclosed a method of initiating production from a gas well by injecting into a subterranean well an aqueous liquid that contains reactants which form nitrogen gas within the well or reservoir, and displaces enough liquid out of the well to lower the hydrostatic pressure and cause fluid to flow from the reservoir to the well. The same reactants produce exothermic heat.
A later patent to RICHARDSON, et al, U.S. Pat. No. 4,399,868, employs the same reactive ingredients as were utilized in the earlier patent to generate nitrogen gas, to not only generate nitrogen gas but, by the exothermic reaction, produce heating of a solvent solution which is introduced into the production zone of a well to effect the dissolution of paraffin and similar deposits from the perforations of the production zone.
These operations are, however, complicated by the presence of a large amount of brine solution which is normally found in a producing zone of a well. Additionally, the solvent solution is heated to temperatures on the order of 440.degree. F. Temperatures of this order of magnitude can be tolerated in the production zone of a well, but would generate excessive stresses if employed in a hydrocarbon transmission conduit, such as a pipeline. More importantly, in such patented processes, the well's ambient temperature was stated to be on the order of 175.degree. F., thus preventing any dissolved paraffins from precipitating out of solution in crystalline form.
U.S. Pat. No. 4,219,083 to RICHARDSON, et al, describes a process for cleaning well casing perforations by injecting an aqueous solution containing nitrogen-gas-generating reactants, an alkaline buffer providing a reaction-retarding pH, and an acid-yielding reactant for subsequently overriding the buffer and lowering the pH in order to trigger a fast-driving pulse of heat and pressure which causes a perforation-cleaning backsurge of fluid through the perforations. Again, this method, as disclosed, might produce undesirable stresses if applied to the cleaning of a pipeline and would not prevent the reprecipitation of the paraffins as the temperature falls, particularly in a subsea pipeline.
There is an obvious need, therefore, for a cleaning method and composition which will minimize the cost of any solvents employed and substantially reduce the cost associated with heating the solvent.